CVE-2026-9816 Insufficient server-side validation of board member role fields permits privilege escalation
Attackers can escalate privileges to board admin by manipulating role fields during member insertion or archive import due to insufficient server-side validation.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9816?
CVE-2026-9816 is a high-severity vulnerability in Mattermost Mattermost Server, published on 17 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685
Weakness: CWE-863
Which versions are affected by CVE-2026-9816?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-9816?
Upgrade to a fixed version.
Is CVE-2026-9816 being exploited?
There is no known exploitation of CVE-2026-9816 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-h2c9-x337-f543, MINI-fp4m-wqwp-rqr4, MINI-xr3m-78c3-9gfg.